1. Field of the Invention
The present invention relates to an engine control system for controlling an engine operating condition such as a fuel injection amount or an ignition timing.
2. Description of the Related Art
Known in the prior art is a D-J type fuel injection system for an internal combustion engine, wherein an intake pressure sensor is arranged in an intake line of the engine at a position downstream from a throttle valve to detect an intake pressure as a parameter of an engine load. The detection of the intake pressure and of an engine speed enables the detection of an amount of intake air fed into the cylinder bore. A fuel injection amount is determined by the detected intake air amount so as to maintain a designated air-fuel ratio value and this amount of fuel is injected by the fuel injector. This D-J type fuel injection system is advantageous in that it makes it possible to mount a smaller sensor and thus reduce the air flow resistance, compared with the L-J type fuel injection system wherein a relatively large air flow meter is arranged in the intake passage for detecting the intake air amount.
Contrary to the L-J type fuel injection system, this D-J type fuel injection system detects an amount of air introduced into the engine indirectly, from the value of the intake pressure. This means that the output level of the sensor has the same value even when the amount of newly introduced air is changed under certain conditions, for example, where only air is introduced into the engine and where a gas, for example, exhaust gas, other than the air is introduced into the engine. Therefore, when an exhaust gas re-circulation operation is carried out, it is necessary to compensate the detected output value of the sensor in order to obtain a correct value of the amount of new air introduced into the engine, if the map is appropriate for an EGR operation. To accomplish this, a system is disclosed in Japanese Unexamined Patent Publication No. 55-75548 wherein a fixed dimension orifice is arranged in an exhaust gas re-circulation passageway, and a pressure sensor is arranged to detect a pressure drop across the orifice. This detected pressure drop is utilized for correcting the output value of the intake pressure sensor, and thus obtain a precise value of the amount of newly introduced air.
This improved system has a drawback, however, in that the precise amount of new air cannot be detected, since it is not possible to directly detect the amount of new air. This has a drawback in that a quick control of the target air-fuel ratio cannot be achieved during a transient state of the engine.